The US secretary of state, Colin Powell, today announced that he had resigned from George Bush's cabinet in a "mutual agreement" with the president.

At a brief press conference, Mr Powell said it had always been his intention to serve only one term and that he and Mr Bush had been talking for a number of months about when he would leave.

"I am pleased to have been part of a team that launched the global war against terror, liberated the Afghan and Iraqi people," he said. Mr Powell also said he "brought the attention of the world to the problem of proliferation, reaffirmed our alliances, adjusted to the post-cold war world and undertook major initiatives to deal with the problem of poverty and disease in the developing world."

Mr Powell submitted his letter of resignation to the president on Friday and today told about 20 of his staff of his plan to depart, according to senior White House sources.

Neither Mr Powell nor the White House would comment today on speculation that Mr Bush had wanted to replace his secretary of state, who was widely seen as a moderate face in a US administration that many, particularly in Europe, regard as hawkish and unilateralist.

Speculation over possible successors to Mr Powell was centred on the US ambassador to the UN, John Danforth, and national security adviser Condoleezza Rice.

Mr Powell, who said he had no plans about what he would do next, said he would go about his usual schedule until a successor was in place. This would be "for a number of weeks or one or two months".

It was not immediately clear whether he would leave before Mr Bush's second inauguration on January 2.

Later today Mr Powell was due to meet the Israeli foreign minister, Silvan Shalom. He heads to Chile tomorrow and then on to Egypt.

Mr Powell is the highest-level official to quit since Mr Bush won re-election, and one of four cabinet members whose resignations were announced today. The three others were Ann Venneman, the agriculture secretary, Rod Paige, the education secretary, and Spencer Abraham, the energy secretary.

Last week saw the resignations of the attorney general, John Ashcroft, and Donald Evans, the commerce secretary. The changes come as Mr Bush embarks on a cabinet reshuffle at the start of his second term in the White House.

Mr Powell has had a controversial tenure as head of the state department, reportedly differing on key issues with the defence secretary, Donald Rumsfeld.

Despite his popularity with foreign statesmen and women, Mr Powell will be remembered for presenting flawed evidence about Iraq's weapons of mass destruction to the United Nations when he made the case for war on behalf of the Bush administration.

Aides close to the secretary of state had indicated in recent months that he would not stay through a second Bush term, and he did not seek to dispel mounting speculation over his possible departure.

His resignation comes just days after he was given a key role in the US diplomatic push to help Palestinians hold elections on a successor to Yasser Arafat.

Palestinian officials say Mr Powell is expected to visit the West Bank next week for talks with the Palestinian caretaker leadership. There has been no official confirmation of the trip from the White House.

Tony Blair paid tribute to Mr Powell, saying: "He is a remarkable man and has been a good friend to this country over a very long period."

The foreign secretary, Jack Straw, speaking at the Foreign Office in London shortly after the announcement, described him as "a very, very fine man, a great soldier, a great statesman". But he said that he was confident of forging a close relationship with whoever Mr Bush chooses to succeed him.

Asked to name the highlights of almost 40 years of public service, Mr Powell said it had always been his policy not to single out any particular moments.

He said that the priorities of his successor would be the war on terror, continuing the establishment of democracy in Afghanistan and controlling the insurgency in Iraq.

Of the Middle East he said it would be seen how good an opportunity for peace had been created by Arafat's death.